Serial recording apparatuses, which record an image by repeatedly scanning a recording head in a direction perpendicular to a direction in which a recording medium such as paper or an OHP sheet is transported, include wire dot, thermosensitive, thermal transfer and ink jet devices.
Of these serial recording apparatuses, the ink jet type records an image by spraying ink directly onto the recording medium, and has the advantage of low operating costs and low noise during recording. Additionally, in the ink jet system of image recording, a certain distance is maintained between the recording head and the recording medium, such that, typically, the two do not contact each other and the ink sprayed from the recording head crosses the space between the recording head and the recording medium so as to reach the recording medium and form a desired image. As a result, the frictional load of the carrier on which the printing head is loaded and scanned can be reduced, making it possible to achieve high printing speeds.
In the case of the above-described ink jet recording head, energy is needed to discharge the ink from the discharge apertures, that is, the nozzles. The amount of this energy varies depending on whether the density of the data to be recorded is high, that is, when a large volume of ink per unit area of the recording medium is discharged, or low, that is, when only a small volume of ink per unit area is discharged.
One method of supplying such required energy involves providing a heating element (that is, a heater) inside each nozzle of the recording head and passing an electric current through the heater so as to generate heat. The heat causes a bubble to form in the ink inside the nozzle, and the nearly instantaneous expansion of the bubble forces the ink out of the nozzle. Such delivery of energy to the recording head, that is, such delivery of electric power to the heater inside the nozzle, is effected via a cable that connects the recording head to the recording apparatus of the main unit. In such a cable, a slight amount of resistance in the wiring itself is present, so the electrical energy supplied via the cable experiences a loss due to that resistance. The size of the loss increases in proportion to the amount of energy supplied, and affects the drive state of the recording head. It should be noted that, in addition to the cable resistance itself, the operating states of the power circuit that supplies direct current power and of other circuit elements as well also change depending on the amount of energy supplied.
For example, in the case of an ordinary ink jet recording apparatus, the wire resistance between the recording apparatus main unit and the recording head is approximately 0.2 Ω and the head contact resistance is approximately 0.1 Ω, so the overall resistance is 0.3 Ω. If a drive current of 100-200 mA per recording element is then supplied and 54 recording elements are driven at the same time, then the overall current totals 2.4 to 4.8A, and the voltage drop due to the wiring also totals 0.3 Ω×(5.4A to 10.8A)=1.62 to 3.24V, which is the voltage fluctuation that is applied to the recording elements.
Naturally, this voltage fluctuation applied to the recording elements translates into fluctuations in the energy with which the ink is discharged from the nozzle, in other words, causes fluctuations in the amount of ink discharged and in the speed at which the ink is discharged. As a result, unevenness occurs in the recording density, gaps arise in the positions at which the drops of ink are discharged onto the surface of the recording medium and sometimes the ink is not discharged properly at all, leading to marked deterioration in the quality of recording.
Additionally, although the voltage applied to the recording elements provided in the individual nozzles of the recording head differs due to the fact that ink is discharged simultaneously from a plurality of nozzles, the drive voltage and drive pulse are set so that the discharge of ink is steady even when ink is discharged simultaneously from a large number of nozzles, that is, when the drive voltage is at its maximum. Accordingly, when ink is discharged simultaneously from a small number of nozzles, the drive voltage and drive pulse applied to the recording elements are excessive, leading to excessive wear on the recording head.
In the typical recording operation, the amount of energy supplied to the recording head varies according to the density of the recorded data as described above, with the result that the accompanying drive states also differ. However, this sort of fluctuation in drive state is an obstacle to the attainment of a uniform recording result. Conventionally, in order to reduce this type of obstacle, a method is used whereby the amount of energy required is calculated and the amount of energy supplied is adjusted to an optimum energy level. It is possible, of course, to obtain the optimum energy amount by measuring physical quantities such as the actual voltage fluctuation, but an easier and more practical method involves counting the number of nozzles from which ink is to be discharged simultaneously using the data that is to be recorded, and from that count calculating the optimum amount of energy.
Moreover, as methods for adjusting the amount of energy supplied, it is possible to vary the drive voltage or to adjust the length of the heating. When changing the drive voltage itself, however, the structure of the circuitry tends to increase in scale, and for this reason it is common to use a drive circuit for the heater and to change the heating period, thereby adjusting the amount of energy supplied.
Additionally, in the recording head described above, the ink is discharged from the nozzle using heat generated by passing an electric current through the heater, so the recording head also generates heat during the process of recording. This increase in the overall temperature of the head is one factor that causes the drive state of the head to fluctuate, and must be taken into account as an element that, together with the above-described recording density, determines the amount of drive energy. Furthermore, differences in individual nozzle performance arising from slight production variations, such as variation in heater resistance value from one nozzle to the next, can also have an effect on the discharge of the ink. Thus the drive state is determined by a wide variety of elements. What is described above represents only the most typical examples, with recording control being exercised by the consideration of these factors to obtain the optimum drive state at any given time and adjust the amount of energy supplied accordingly, in order to obtain better-quality recording results.
Additionally, as personal computers (hereinafter sometimes referred to simply as PCs) have become faster, it has become possible to more easily handle large volumes of color image data, such that it is preferable to process large amounts of data when recording color images as well. Furthermore, the increasing fineness of recording images and increasing speed of processing makes it necessary to process ever larger amounts of image data at high speed. Increasing the speed of the recording operation in a serial-type ink jet recording apparatus like that described above can be achieved by increasing the number of cycles during which ink is discharged from the nozzles and by increasing the number of nozzles on the recording head. Enhanced fineness of the recorded image can be achieved by packing the recording head nozzles more densely together. However, such configurations tend to result in increasing numbers of nozzles to be driven per unit of time, and by increasing the number of nozzles to be driven per unit of time the number of nozzles involved in discharging ink simultaneously also increases, resulting in an increase in fluctuations in the drive state due to recording density as described above.
Additionally, in order to obtain highly detailed recording images, it is foreseeable that the degree of resolution required will differ depending on the recording color and contents. For example, there are cases in which it is best that the ink drops to be used for recording a photographic image differ from the ink drops to be used for recording an image that consists primarily of text. Accordingly, the same ink jet recording apparatus may have a plurality of recording heads of different resolutions. In a case in which a plurality of recording heads of different resolutions are used simultaneously, the timing of the discharge of the ink from the individual heads differs depending on the arrangement of the nozzles and the frequency with which the ink is discharged with respect to the distance over which the head is scanned. Also, as the size of the drops of ink discharged from the nozzles increases, so, too, does the amount of energy required to discharge the ink, with the result that the length of time required to heat each heater of each nozzle in order to discharge one drop of ink from an individual nozzle differs with each recording head.
Given these reasons, in the above-described structure, when an effort is made to calculate the number of nozzles driven simultaneously in a plurality of recording heads, because the individual recording heads are driven at different times it is difficult to determine how many nozzles are being driven at any one time. Additionally, when considering the energy supply side of the matter, in order to keep the cost of the device low it is necessary to supply electrical power to the individual recording heads using a single source of power. As a result, although it is necessary to determine the optimum amount of energy to be supplied not just to the recording density (drive state) of one recording head but to each of the several recording heads while taking into consideration the drive states of every other recording head, it has not been easy to do so.
(1) It has not been possible to independently determine the voltage drop generated by the driving of the recording heads, the amount of the pulse current voltage drop in the path of the power wiring for the recording head and the voltage drop due to the smooth drive current that changes relatively smoothly.
(2) In the recording head, which is composed of a plurality of chips (head substrates), it has not been possible to independently determine the extent of the voltage drop in the wiring region common to all chips and the extent of the voltage drop in the individual wiring region of each chip.
Due to such problems, it has not been possible to determine accurately the extent of the voltage drop in the timing that drives the recording element.
At the same time, newer recording heads, which seek to achieve greater recording speeds by increasing the number of recording elements therein, also tend to continually increase the number of such recording elements that are driven at the same time. Accordingly, in order to accurately determine the extent of the voltage drop there is an increasing need to secure stable discharge of ink by performing appropriate pulse control.